Method of improving recovery of neptunium in the purex process

ABSTRACT

The recovery of neptunium values from the Purex Process for reprocessing irradiated nuclear reactor fuel is improved by adding a rate-accelerating material to increase the rate of oxidation of the neptunium in the nitric acid solution in the first extraction column. The rate-accelerating material is formed by adding 1-nitropropane to a sodium hydroxide solution to form the aci-form of nitropropane, then adding sodium nitrite and acidifying the solution by the addition of nitric acid.

United States Patent Swanson [451 Mar. 28, 1972 [54] METHOD OF IMPROVINGRECOVERY OF NEPTUNIUM IN THE PUREX PROCESS [72] Inventor: John L.Swanson, Richland, Wash.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission [22] Filed: Dec. 29, 1969 [21] Appl.No.:888,831

3,004,823 l0/l96l Peppard et a] ..23/34l 3,326,811 6/1967 Heaiy......23/341 3,432,276 3/1969 Reas ..23/343 OTHER PUBLlCATIONS Bruce et al.,ed., Progress in Nuclear Chemistry Series III, Process Chemistry, Vol.3, Pergamon Press, N.Y., 1961, p. 255.

Primary Examiner-Carl D. Quarforth Assistant Examiner-F. M. GittesAttorney-Roland A. Anderson V W ABSTRACT The recovery of neptuniumvalues from the Purex Process for reprocessing irradiated nuclearreactor fuel is improved by adding a rate-accelerating material toincrease the rate of oxidation of the neptunium in the nitric acidsolution in the first extraction column. The rate-accelerating materialis formed by adding l-nitropropane to a sodium hydroxide solution toform the aci-form of nitropropane, then adding sodium nitrite andacidifying the solution by the addition of nitric acid.

6 Claims, No Drawings METHOD OF IMPROVING RECOVERY OF NEPTUNIUM IN THEPUREX PROCESS CONTRACTUAL ORIGIN OF THE INVENTION The inventiondescribed herein was made in the course of, or under, a contract withthe United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to an improvement ina process for separating neptunium from uranium, plutonium and fissionproduct values and more particularly relates to an improvement in thePurex Process for the processing of irradiated nuclear reactor fuels andfor increasing the recovery of neptunium values therefrom. Neptuniumrecovery from the Purex Process is discussed in Neptunium Recovery andPurification at Hanford by R. E. Isaacson and B. F. Judson, I & ECProcess Design and Development, Vol. 3, No. 4, Oct. 1964, page 296.

Np not only has utility as a research isotope but also as a source of PuThe latter isotope has become important to the general field of spaceexploration as a heat source for power units.

Neptunium is produced by either of the following reactions:

v) 'r) (B) U Since either U or U is present in most reactors, Np will beproduced, along with various fission products, during fuel burnup.Neptunium exists in the +4, +5 and +6 valence states but is extractablefrom aqueous solutions with alkyl phosphates only when in either the +4or +6 state.

The Purex Process is a solvent extraction process used in reprocessingirradiated fuel elements fabricated of normal or slightly enricheduranium. The organic solvent employed is 30 percent tributyl phosphate(TBP) in normal paraffin hydrocarbon (NPH). The salting agent is nitricacid.

In the Purex Process, the irradiated fuel elements are dissolved innitric acid to form a feed solution. The feed solution then enters thefirst cycle of the solvent extraction system where a gross separation iseffected between the fission products and theuranium/neptuniumlplutonium mixture. This is accomplished by counterflowwith the TBP organic solvent. The fission products remain in the aqueousphase, the uranium/plutonium and neptunium values transferring to theorganic phase. In a second cycle of treatment, the uranium and plutoniumare separated. The neptunium values separate with the uranium from theplutonium and the plutonium is further processed to remove fissionproducts. The neptunium and uranium values are separated by contactingthe acid wherein they are contained with an organic extractant whichremoves the uranium values, leaving the neptunium values in the acidsolution for further concentration and recovery.

In order to extract the neptunium values along with the plutonium valuesfrom the acid solution in the first cycle, the neptunium values must beoxidized to the +6 valence state. This oxidation is presentlyaccomplished in the process by adding HNO to the nitric acid feedsolution which acts to catalyze the neptunium oxidation. However, thePurex Process is a continuous process and residence times in theseparation columns are not sufficient to permit complete oxidation ofthe neptunium values present in the column to the extractable +6 valencestate. Thus a significant percentage of the neptunium values present inthe nitric acid feed solution are presently not extracted. Theunextracted neptunium values are lost in the aqueous waste solution tothe storage tanks, although some may be recovered later by additionalprocessing of the waste solutions.

SUMMARY OF THE INVENTION I have invented an improvement in the processof extracting neptunium values from a nitric acid nuclear fuel feedsolution containing neptunium values, including neptunium in the +5valence state, with tributyl phosphate wherein there is added to saidfeed solution a material to increase the rate of oxidation of neptuniumhaving a +5 valence state to a neptunium having a +6 valence state, saidmaterial being formed by adding l-nitropropane to an aqueous solutioncontaining dilute NaOH thereby forming the aci-form of nitropropane,adding sodium nitrite to said solution and adding nitric acid to acidifysaid solution. The rate-accelerating material when added to the nitricacid feed solution, since it increases the rate at which oxidation ofthe neptunium occurs in the solution, thus increases the amount ofneptunium in the +6 valence state W available for extraction. Myinvention is particularly applicable in the Purex Process resulting ingreatly improved recoveries of neptunium values. My improvement asapplied to the Purex Process consists of adding a material, called arate-accelerating material, to the Purex Process during the first cycleseparation. This material increases the rate at which the neptuniumvalues are oxidized from the inextractable +5 valence state to theextractable +6 valence state, thus oxidizing more of the neptuniumpresent in the feed solution during the period of residence in theextraction column. This greatly increases the amount of neptunium valueswhich may be extracted from the acidic feed solution and which may thenbe separated from the co-extracted uranium and plutonium values bysubsequent processing. By my process, the rate-accelerating material ismade by adding l-nitropropane to a dilute solution of NaOH to form theaci-form of lnitropropane, then adding sodium nitrite and acidifying thesolution by the addition of nitric acid. The rate-accelerating materialis then added to the first cycle extraction column where it increasesthe oxidation rate of the neptunium values present in the nitric acidfeed solution, thus permitting more complete extraction of these valuesand improving the recovery thereof.

It is therefore an object of this invention to improve the recovery ofneptunium values from the nitric acid feed solution in the PurexProcess.

DESCRIPTION OF THE PREFERRED EMBODIMENT These and other objects may beachieved by making the acid feed solution from 0.005 to 0.02 M in addednitropropane which is made by adding up to 2 M of lnitropropane to anaqueous solution containing up to 2 M NaOH to form the aci-form ofl-nitropropane, adding sodium nitrite to the solution in a ratio of 0.3to 0.6 nitrite to lnitropropane and acidifying the solution by addingsufficient nitric acid so that the final solution contains from about 3M to 8 M of nitric acid. Because the composition of the resultingproduct, which is the rate-accelerating material (RAM), is unknown, theamount of material to be added to the acid feed solution is determinedby reference to the concentration of lnitropropane which would have beenpresent had it not been consumed in the formation of the RAM. This isreferred to as added nitropropane. Thus sufficient quantity of thesolution is fed into the first column so that the acid feed solutioncontains about 0.01 M of the added nitropropane. The presence of theadded nitropropane in the column accelerates the oxidation rate of theneptunium present therein, thus causing a high percentage of the totalneptunium present to oxidize to the extractable +6 valence, permittingextraction of more of the neptunium from the acid feed solution into theorganic extractant. The neptunium values are then separated from theuranium and plutonium values which are also extracted and furtherpurified.

The aci-form of l-nitropropane is prepared by the addition ofl-nitropropane to an aqueous solution of NaOH. Solutions havingconcentrations of l-nitropropane as high as 2 M may be prepared. Theamount of NaOH used is the same as, or just slightly in excess of, thatrequired to convert the nitropropane to the aci-form.

The sodium nitrite may be added to the sodium hydroxide solution eitherbefore or after the addition of the lnitropropane. The amount of sodiumnitrite varies with the 1- nitropropane present, with a ratio of nitriteto l-nitropropane of 0.3 to 0.6 being operable. However, 0.4 ispreferred as the best results are achieved with this ratio.

After formation of the aci-form of l-nitropropane, the solution with theaci-form and the nitrite is acidified by mixing with nitric acid to formthe rate-accelerating material. The concentration of HNO in the finalsolution may vary from 3 to 8 M, although the preferred concentration is7 M. Sufficient acid must be mixed with the solution containing theaci-form and sodium nitrite to account for dilution of the acid and anyneutralization which may occur due to excess NaOl-l present.

it is preferred that the aci-form solution and the nitric acid be fedinto a reactor simultaneously while mixing the solutions well andcontinuously withdrawing the product formed. The solution containing theRAM is immediately added to the nitric acid feed solution in theextraction column.

Residence time of the solutions in the reactor necessary for formationof the rate-accelerating material is found to be dependent upon thetemperature at which the mixing occurs. Thus good production of RAM wasobtained with a residence time of 2 minutes at 35 C. At temperaturesmuch over 36 C. for this residence time, the amount of product producedbegan to diminish. Almost as good results were achieved with a residencetime of about 9 minutes at a temperature of about 30 C. Again for thistime period, temperature variance without a time change caused adecrease in product formed.

Agitation or stirring speed at which the solutions were mixed was alsofound to affect the amount of RAM produced. Thus sufficient agitation isnecessary to ensure good mixing of the solutions, while too muchagitation will cause a decrease in the amount ofproduct formed.

Sufficient RAM is added to the first cycle column in the Purex cyclewhere the organic extractant contacts the nitric acid feed solution (HAcolumn) so that the aqueous nitric acid feed phase contains from 0.005to 0.02 M in added nitropropane. While lower concentrations wouldaccelerate the oxidation sufficiently to increase yields of neptuniumvalues, the RAM is unstable due to the radiation present in the columnso the higher concentrations are preferred to ensure essentiallycomplete oxidation of neptunium. Concentrations greater than 0.02 M willincrease the waste volumes unnecessarily, thus adding to the cost of theoperation.

Formation of the rate-accelerating material is apparently possible onlywith primary nitroalkanes. The formation of RAM was attempted with theaci-form of 2-nitropropane and 2-nitrobutane, but no increase in theoxidation rate of neptunium was noted from the product obtainedtherefrom.

The identity of the RAM is unknown and it may be one or more compounds.Based on the sensitivity of the RAM formation to preparative conditions,it is believed that RAM is an intermediate in, or a byproduct of, one ormore of the reactions known to occur on the acidification of theaci-form of a primary nitroalkane. These are (a) conversion to the truenitroalkane, (b) conversion to the aldehyde and N and (c) conversion tothe nitrolic acid followed by conversion to the carboxylic acid.

The following examples are given as illustrative of the process of theinvention and are not to be taken as limiting the scope or extent of theinvention.

EXAMPLE I To determine the effect of RAM on the Np(V) oxidation state aseries of experiments were run. The solutions contained 3.1 M HNO;,, X10 M NaNO and varying amounts of l-nitropropane at about 24 C. From theresults of the experiments, the one-half times listed in the table belowwere calculated.

[RNOz] kzlIRAlVfl/[RNOzl k2[RAM] til: (min) 30 0. 1x10 1 7 1. 0X10 l0 0.8 0. 1X10 10 063 RNO2=concentration oi nitropropane which would havebeen resent had it not been condensed in the formation of RAM.

in RAM1=a measure of the desired property-that of making Np(V) oxidizemore rapidly.

t =time necessary for one-half of the Np(V) present in the solution tooxidize to N )(VI) As can be seen from the above results, the effect ofthe RAM is to greatly decrease the time necessary to oxidize the Np(V)values present to the extractable N p(Vl) state.

EXAMPLE 11 0.5 M of l-nitropropane was added to an aqueous solutioncontaining 0.6 M NaOH and 0.2 M sodium nitrite. 5.2 ml./min. of thissolution were fed along with l 1.4 ml./min. of 11.0 M HNO;, solutioninto a stirred reactor having a volume of 32 ml. at a temperature of 37C. These values gave a solution composition of 0.16 M added nitropropaneand 7.4 M HNO,,. The residence time in the reactor was 2 minutes.sufficient quantity of this solution was added to a 3.1 M HNO solutioncontaining 5 X 10 sodium nitrite and S X 10 M neptunium values to makethe solution 10' M in added nitropropane. The one-half time, which isthe time required for the reaction to proceed half of the way to theequilibrium position, was found to be 1.1 minutes at 25 C. This compareswith a similar experiment which varied only in containing no RAM whereinthe one-half time of neptunium oxidation was 30 minutes.

It can readily be seen from the above experiments that the addition ofRAM has a profound effect upon the rate of oxidation of neptunium valuesfrom the +5 to the +6 valence state.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The process of increasing the amount of neptunium vales extractableby tributyl phosphate from a nitric acid feed solution of irradiatedfuel elements fabricated of normal or enriched uranium and containingneptunium in the +5 valence state, uranium, plutonium and fissionproducts which comprises making the feed solution from 0.005 to 0.02 Min nitropropane by the addition of an oxidation rate acceleratingmaterial made by adding up to 2 M of l-nitropropane to an aqueoussolution containing up to 2 M NaOH to form the aciform l-nitropropane,adding sodium nitrite to the solution in a ratio of 0.3 to 0.6 parts ofsodium nitrite to one part of lnitropropane and acidifying the solutionby adding sufficient nitric acid so that the final solution is fromabout 3 M to 8 M in nitric acid and contacting said nitropropanecontaining feed solution with an organic extractant of 30 percenttributyl phosphate in normal paraffin hydrocarbon, thereby extractingthe uranium, plutonium and neptunium values.

2. The process of claim 1 wherein the NaOH is present in slight excessof that necessary to convert the --nitropropane to the aci-form.

3. The process of claim 2 wherein the NaOH solution is 0.6 M in NaOH,0.5 M in l-nitropropane, 0.2 M in sodium nitrite, the solution is made 6M in HNO and a sufficient amount of this solution is added to the nitricacid feed solution to make it 0.02 M in nitropropane.

4. In the process for separating neptunium values from a nitric acidfeed solution containing neptunium in the +5 valence state, uranium,plutonium and fission product values by adding nitrous acid to the feedsolution to oxidize the neptunium values from the inextractable +5valence state to the extractable +6 valence state, contacting the acidfeed solution with an organic extractant of 30 percent tributylphosphate in normal paraffin hydrocarbon, thereby extracting theuranium, plutonium and neptunium values, and separating the neptuniumvalues from the uranium and plutonium values, the improvement in theoxidation of the neptunium values comprising: making the nitric acidfeed solution from 0.005 to 0.02 M in nitropropane by the addition of anoxidation rate accelerating material said material formed by adding upto 2 M 1- nitropropane to an aqueous sodium solution containing up to 2M sodium hydroxide to form the aci-form of nitropropane, adding sodiumnitrite to said solution in a ratio of 0.3 to 0.6 parts of sodiumnitrite to one part of l -nitropropane and acidifying the solution byadding sufficient nitric acid, so that the final solution is about 3 Mto 8 M in nitric acid thereby forming the rate-accelerating material,said material when added to the nitric acid feed solution therebyincreasing the rate at which said oxidation of said neptunium occurs insaid solution, thus increasing the amount of neptunium in the +6 valencestate available for extraction.

5. The process of claim 4 wherein the NaOH is present in slight excessof that necessary to convert the l-nitropropane to the aci-form.

6. The process of claim 5 wherein the NaOH solution is 0.6 M in NaOH,0.5 M in l-nitropropane, 0.2 in M sodium nitrite, the solution is made 6M in HNO and a sufficient amount of this solution is added to the nitricacid feed solution to make it 0.02 M in nitropropane.

2. The process of claim 1 wherein the NaOH is present in slight excessof that necessary to convert the 1-nitropropane to the aci-form.
 3. Theprocess of claim 2 wherein the NaOH solution is 0.6 M in NaOH, 0.5 M in1-nitropropane, 0.2 M in sodium nitrite, the solution is made 6 M inHNO3 and a sufficient amount of this solution is added to the nitricacid feed solution to make it 0.02 M in nitropropane.
 4. In the processfor separating neptunium values from a nitric acid feed solutioncontaining neptunium in the +5 valence state, uranium, plutonium andfission product values by adding nitrous acid to the feed solution tooxidize the neptunium values from the inextractable +5 valence state tothe extractable +6 valence state, contacting the acid feed solution withan organic extractant of 30 percent tributyl phosphate in normalparaffin hydrocarbon, thereby extracting the uranium, plutonium andneptunium values, and separating the neptunium values from the uraniumand plutonium values, the improvement in the oxidation of the neptuniumvalues comprising: making the nitric acid feed solution from 0.005 to0.02 M in nitropropane by the addition of an oxidation rate acceleratingmaterial said material formed by adding up to 2 M 1-nitropropane to anaqueous sodium solution containing up to 2 M sodium hydroxide to formthe aci-form of nitropropane, adding sodium nitrite to said solution ina ratio of 0.3 to 0.6 parts of sodium nitrite to one part of1-nitropropane and acidifying the solution by adding sufficient nitricacid, so that the final solution is about 3 M to 8 M in nitric acidthereby forming the rate-accelerating material, said material when addedto the nitric acid feed solution thereby increasing the rate at whichsaid oxidation of said neptunium occurs in said solution, thusincreasing the amount of neptunium in the +6 valence state available forextraction.
 5. The process of claim 4 wherein the NaOH is present inslight excess of that necessary to convert the 1-nitropropane to theaci-form.
 6. The process of claim 5 wherein the NaOH solution is 0.6 Min NaOH, 0.5 M in 1-nitropropane, 0.2 in M sodium nitrite, the solutionis made 6 M in HNO3 and a sufficient amount of this solution is added tothe nitric acid feed solution to make it 0.02 M in nitropropane.